1. Field of the Invention
The present invention relates to the use of nonionically modified, water-dispersible polyisocyanates having aliphatically and/or cycloaliphatically bound isocyanate (groups as conditioners for inorganic binders in the production of high-density/high-strength mortar or concrete compositions.
2. Description of the Prior Art
Conventional concrete/mortar compositions contain an inorganic binder such as cement; an aggregate such as gravel or sand; water; and also optionally further conditioners and/or additives. The only difference in building material terms between concrete and mortar in this context is the maximum particle size of the aggregates used. When the maximum particle sizes are up to 4 mm, the mixture is generally referred to as mortar, while with coarser aggregates, the mixture is generally referred to as concrete. The terms "concrete" and "mortar" will not therefore be further differentiated in the context of the present invention, the two terms instead standing synonymously for any mineral building materials based on inorganic binders.
Plastics are today increasingly used as conditioners for improving the working and service properties of concrete, for example, to increase strength or chemical resistance. H. Schorn, Betone mit Kunststoffen, [Concretes Containing Plastics] Ernst & Sohn Verlag fur Architektur und technische Wissenschaften, Berlin 1991, p. 25, for example, gives an overview of the wide variety of polymers which have already been tried out for modifying concrete.
Polyisocyanates have also already been proposed as possible concrete conditioners. Aromatic polyisocyanates, such as diphenylmethane diisocyanate (MDI), tolylene diisocyanate (TDI) or derivatives thereof, are described in DE-A 1,924,468 as suitable conditioners for cement mixtures. Biurets and urethanes of the aliphatic hexamethylene diisocyanate (HDI) have also been disclosed for this purpose. When these polyisocyanates, either alone or combined with other isocyanate-reactive polymers, are admixed with a concrete composition in the manufacture of floor coverings, accelerated complete hardening is said to result such that the floor coverings may be walked on sooner. EP-A 23,579 also discloses the effect of more rapid full hardening of concrete/mortar compositions following the addition of small quantities of organic, preferably aromatic, polyisocyanates.
In both of these references exclusively hydrophobic polyisocyanates are used which are completely incompatible with water and cannot be stirred homogeneously into an inorganic binder even when considerable quantities of organic solvents are co-used. However, as uniform as possible a distribution of the polyisocyanate component in the binder mixture is a fundamental prerequisite for forming a uniform polymer skeleton within the inorganic binder matrix and, thus, for achieving optimum final properties in the concrete.
EP-A 181,261 describes the use of finely divided aqueous dispersions of polyisocyanates for the endogenous carbonatization of concrete. The only polyisocyanate used in this case is hydrophobic MDI which, premixed with water in an weight ratio of from 4:1 to 2:1, i.e., in the form of a water-in-oil emulsion, is added to the concrete composition using a high-speed stirrer. Although the water-in-MDI emulsions obtained with the use of high shear forces show a limited stability in isolation, it can be demonstrated that when the latter emulsions are stirred into an excess quantity of an aqueous system, such as a concrete mixture, phase separation takes place instantaneously, and no finely divided oil-in-water emulsions are obtained. According to the process of EP-A 181,261, it is also not possible to incorporate polyisocyanates in concrete or mortar compositions in a sufficiently homogeneous manner.
According to the teachings of DE-A 2,300,206, the compatibility of hydrophobic polyisocyanates with cement compositions can be improved markedly by combining them with water-soluble polymers, for example cellulose derivatives, polyvinyl alcohols or polyether alcohols, which optionally also contain additional emulsifiers. However, the resulting aqueous cement mixtures remain workable only for impracticably short periods of a few minutes, even when organic solvents are co-used. In addition, the considerable quantities of water-soluble polymers which are introduced into the binder provide permanent hydrophilic properties, which result in concrete having undesirably high water absorption.
U.S. Pat. No. 4,143,014 describes a highly specific process for incorporating a hydrophobic polyisocyanate in an aqueous system. According to this process, mixtures of water-soluble polyether diols with an excess, based on equivalents, of MDI can, within a short period following the onset of the urethanization reaction but still before its termination, be stirred in to form clear, stable solutions in water. Such solutions, according to U.S. Pat. No. 4,228,053, are said also to be suitable for improving the strength and resistance properties of concrete. The precise point in time from which the reaction mixtures prepared from MDI and polyether alcohol show sufficient water-miscibility for a few minutes is dependent on the type of polyether, diol utilized and can be determined only by laborious preliminary experimentation. Therefore, this process cannot be used in commercial operations.
All of the preceding processes for modifying concrete by the addition of polyisocyanates either require the use of additional organic compounds, for example, solvents or special water-soluble polymers, which raise ecological concerns, or they require special mixing units such as high-speed stirrers in order to render the hydrophobic polyisocyanate component mixable with the inorganic binder even to a minor degree.
Attempts have already been made to avoid these disadvantages by utilizing self-dispersible polyisocyanates, which have been hydrophilically modified by the incorporation of ionic groups (DE-A 2,359,611), as conditioners for inorganic binders. Such polyisocyanates containing salt groups can be stirred in very finely divided manner into aqueous systems without the need for high shear forces, but their stability in storage is completely inadequate. Even at room temperature, the known catalytic activity of ionic groups gives rise to the polymerization of the isocyanate groups, for example, by trimerization into polyisocyanurates or with the formation of .alpha.-nylon structures, which results in gelation of the product, generally within a few days.
An object of the present invention is now to provide novel isocyanate-functional conditioners to improve the working and service properties of concrete, which do not have the disadvantages of the prior art conditioners. These novel conditioners should be stable in storage and free of organic solvents, should be readily stirrable into aqueous systems without the need for special mixing units, and should provide dispersions having the smallest possible particle size and greatest stability.
The isocyanate/water reaction should proceed as slowly as possible in these dispersions to ensure that the time in which the concrete compositions remain workable is adequate.
This object may be achieved with by using water-dispersible polyisocyanates, which are known, as conditioners for inorganic binders. The invention, which is described in greater detail hereinafter, is based on the surprising observation that the addition to conventional concrete or mortar compositions of even only very small quantities of water-dispersible polyisocyanate blends, which have been rendered hydrophilic with nonionic groups, can bring about a considerable improvement in the mechanical properties, such as compressive, tensile strength and elasticity. It is particularly surprisingly that despite the fact that CO.sub.2 arises within the concrete composition during the isocyanate-water reaction and unavoidable endogenous carbonatization consequently takes place, no negative effects on the strength and resistance properties are discernible.